EMI Shielding

EMI Shielding

The Dexmet MicroGrid® EM series of expanded metal foils (EMFs) are versatile, effective materials for shielding against electromagnetic interference (EMI). EMFs are formed from solid metal foil in a proven, economical “slit-and-stretch” process. They are electrically continuous and, unlike knit or woven meshes, exhibit consistent and predictable conductivity. Thin, strong, flexible and lightweight, EMFs will not fray or unravel, and conform readily to complex surfaces, making them well-suited to composite manufacturing processes.

Standard configurations include copper, aluminum, nickel and Monel meshes at .002 and .003″ thick (50 and 75µm). Dexmet can readily produce custom types using other metals and foil thicknesses, including Cu and Al types down to .001″ (25µm). The open area of the mesh can be precision-tailored to meet user requirements for weight, resistivity, formability and shielding effectiveness.

Applications:

  • Shielded enclosures
  • Gaskets
  • Ventilation screens
  • Cable shielding
  • Laminated structures
  • Cockpit electronics
EMI Shield Image

Standard configurations:

Weight2 Shielding Effectiveness1(dB)
Type Material Thickness g/in2 g/m2 Open area (%) 100 MHz 1 GHz 10 GHz
EM2Cu Cu .002″ (50µm) .139 215 53% 72 53 33
EM3Cu Cu .003″ (75µm) .158 245 64% 60 42 25
EM2Al Al .002″ (50µm) .042 65 53% 70 51 32
EM3Al Al .003″ (75µm) .048 74 64% 58 41 23
EM2Ni Ni .002″ (50µm) .138 214 53% 60 46 28
EM3Ni Ni .003″ (75µm) .157 243 64% 54 40 24
EM2ML Monel .002″ (50µm) .175 271 39% 67 53 36
EM3ML Monel .003″ (75µm) .255 395 41% 63 46 30

1Test method: ASTM D4935-10
2±10

Design Factors

The key measure of merit for EMI shielding materials is “shielding effectiveness” (SE). SE is a measure of how well a material reduces (attenuates) electromagnetic field strength.

The SE of expanded metal foils is dependent on several factors, including:

  • Material type
  • Foil thickness
  • Size of openings
  • Openings per unit area

These parameters can be controlled during manufacturing to yield meshes custom-tailored for weight, resistivity and shielding effectiveness.

Download our Information Note (Below) to learn how shielding effectiveness is measured, and how MicroGrid® EM expanded foils can be tailored to your requirements.

se_doc

 

 

Technical References

  1. Mesh Per Inch (MPI)

    EMI RFI Shielding Grid

    Measure one inch and count the number of meshes (or openings – between two nodes) along the SWD direction. We generally refer to MPI in the SWD direction and not the LWD direction. See Standard Product Range for the mean MPI count for each Mesh Designation.

  2. Openings Per Square Inch

    EMI RFI Shielding Grid 2

    Double the product of MPI and LWD mesh count. Openings per square inch = (3.5 x 2) x 2 = 14

  3. Product Code Nomenclature

    Product Example: 3 Ni 5-077

    • Original Foil Thickness: 0.003 in.
    • Metal or Alloy: Nickel
    • Strand Width: 0.005 in.
    • – LWD: 0.077 in.

  4. Coverage Area Calculation

    (Accurate to ±10%)
    Coverage Area = 2 x MPI x Strand Width
    Product Example: 2 x 24 x 0.005 = 0.24 Coverage Area = 24%

  5. Open Area Calculation

    (Accurate to ±10%)
    Open Area = 1 – Coverage Area
    Product Example: 1 – 0.24 = 0.76 Open Area = 76%

  6. Weight Per Area Calculation

    (Refer to Density of Materials Chart)
    Grams per square inch = metal weight (Lbs per cubic foot) ÷ 12 x original foil thickness x 2 x MPI x strand width x 3.1416
    Product Example: 554.688 ÷ 12 x 0.003 x 2 x 24 x 0.005 x 3.1416 = 0.10456 grams/sq. in.

  7. Total Surface Area Calculation

    A = Coverage Area
    B = 85% of Original Foil Thickness
    C = B/Strand width
    D = Total Surface Area = 2A (1+C)
    Product Example: A = 0.24

    B = 0.85 x 0.003 = 0.00255
    C = B/0.005 = 0.00255/0.005 = 0.51

    Total Surface Area = 2 x 0.24 x (1+0.51) = 0.7248 sq. ft. of Surface Area per sq. ft. of MicroGrid®

  8. Resistivity and Conductivity

    See Resistivity and Conductivity Chart

Physical Testing of Expanded Metal Foil

The first rule in testing expanded metal and expanded plastic is the need for new rules. The standards of Ultimate Tensile, Yield Strength and Elongation do not necessarily apply. Simply establishing the area to be considered becomes a point of discussion. The solution is to eliminate the questionable areas and concentrate on what we know to be true. The tests are simple and do not generally require expensive equipment, so quality may be verified by our customers.

Tensile Testing

Tensile Testing to failure (Ultimate Tensile) is usually more of a shear test than a tensile test. Tensile Testing rips rather than pulls the strands apart. Depending on the configuration of the product, pulling the mesh may actually change it from one form to another. For example, a flattened metal foil will revert to its expanded configuration before failing.

Bend Testing

Dexmet has established a simple yet effective test that utilizes the elasticity of the material. As the material is annealed, its elasticity, or spring-back, decreases. Relative stiffness can therefore be translated into degrees of anneal. A history has been developed that tells us whether we have accomplished a complete anneal. Feedback from our customers has also provided the specific numerical hardness requirements for a variety of processes. We know that certain rolling mill practices can adversely affect the ability of material to function in our process as well as in our customer’s applications. Testing has also shown that stress relieving will make the material feel softer and lay flatter but will not affect the bend recovery.

Pull Testing

Pulling to within a small percentage of what would result in permanent deformation gives a reasonable measure of the force the material can tolerate without undue stretching. However, if stretching must be avoided entirely, pulling to the same distance a second time will result in the amount of force that the material can withstand without permanent deformation. This information can be very useful to the machine designer.

Salt Solution Plating Tests

The immersion of a sample of Nickel plated steel mesh in a bubbling 2% salt solution is an excellent test for plating quality. There is sufficient air to readily oxidize the nickel, but it is controlled. Salt Solution Testing remains one of the best methods for evaluating the post plating process. Exposure time provides a means to quantify the results.

The chart below displays the shielding effectiveness (SE) of Dexmet MicroGrid® EM foils at frequencies from 30 MHz to 8 GHz 1.

Dexmet has compiled SE data on dozens of expanded mesh types. Contact us for measurement data for other materials, thicknesses and open areas.

shieldingchart2

1Test method: ASTM D4935-10

Contact Us

Phone: (800) 714-8736
Fax: (203) 294-7899

sales@dexmet.com

22 Barnes Industrial Rd. So.
Wallingford, CT 06492
United States of America

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